This invention relates to the art of radio receiver multicouplers, and more particularly to a new and improved multicoupler having multiple, isolated outputs from a single input which is practical for very short wavelength operation in the microwave region and all lower frequencies.
Receiver multicouplers are radio communications products which produce a number of output ports from a single input and which have isolation of some degree between output ports. These multicouplers may be: passive, having power loss through a power divider; active, having an amplifier preceding the lossy power divider to produce positive overall gain; or completely electronic, having amplifiers which provide the distribution and isolation function. Each method has certain disadvantages. The all-electronic systems exemplified by U.S. Pat. Nos. 3,710,360, 3,392,335 become less useful at frequencies above 50 MHz. The power divider system, whether passive or active, is in universal use at present above 30 Mhz. Power divider output levels are always attenuated in proportion to the number of ports. To overcome this attenuation which reduces the following receiver's sensitivity, an amplifier usually precedes the divider. Power divider systems have large losses for large number of outputs, requiring larger amplifier gains, and are bulky and not easily field expandable. Use of power dividers in very weak signal environments is tolerated because of the special requirements imposed by the standard use of low impedance cable for interconnection at radio frequencies. This impedance is under 100 ohms, generally 50 ohms.
Previous attempts to modify this power divider method have used various means to distribute the signal other than by a power divider. In these alternate methods, the input amplifier is still used but the power divider is replaced by amplifier circuits or transformers to produce some degree of isolation between output ports. Isolation between output ports is required to keep ports independent of each other. The principle common to all of these alternate methods is that an amplifier matches the input impedance and it or another amplifier acts as a buffer or impedance changer, in addition to providing gain. All distribution is performed in the output stage of this amplifier where there is no concern of effects on the input. At higher frequencies, the problem of space asserts itself. The problem of equal distribution to separated ports, physically distant from each other by inches from a single point in the output of an amplifier, is difficult when these distances are a large fraction of the signal wavelength. These methods therefore work well at low frequencies but are not practical above 50 MHz.
It would, therefore, be highly desirable to provide a receiver multicoupler which operates in a different manner, is simpler, provides greater isolation and is practical at high radio frequencies.